1. Field of the Invention
This application relates to an alarm device for a wet clutch in which an input shaft is connected to or disconnected from an output shaft by charging or discharging hydraulic oil (fluid) pressure into or out of an oil pressure chamber in the clutch (a clutch oil chamber), whereby the alarm device raises an alarm as to the duration of life (a lifetime prediction alarm) of the clutch.
2. Background of the Invention
The patent reference 1 (JP1993-164149) discloses a method for determining criteria as to the duration of life (a use limit regarding operating time) of a wet clutch, the clutch being provided between an engine and a driven machine such as a forklift, and a clutch disk of the clutch being connected to or disconnected from a counter clutch disc of the clutch so that an input shaft is connected to or disconnected from an output shaft by charging or discharging hydraulic oil (fluid) pressure working on the surfaces of the clutch discs.
In the technology of the patent reference 1, the main control unit for the clutch is provided with:                a clutch control means to control the clutch via an actuator;        an input rotation speed detecting means to detect rotation speed for the input shaft of the clutch;        an output rotation speed detecting means to detect rotation speed for the output shaft of the clutch;        an actuator position detecting means to detect the position of the actuator where the actuator is shifted by a supposed displacement from the actuator final position where the actuator is set free; and        a clutch wear estimating means for judging the clutch to be worn, through an arithmetic operation as to the slip of the clutch based on the input rotation speed and the output rotation speed, when the result of the arithmetic operation as to the slip reaches a predetermined value; wherein,the clutch wear judgment is performed by use of the result of the arithmetic operation as to the slip, when the position of the actuator reaches to the predetermined a position predetermined by the actuator position detecting means.        
The patent reference 2 (JP3348590) discloses a remaining lifetime estimating device for a wet clutch, whereby a clutch disc temperature Tp is calculated based on a revolution speed difference ΔN, a control oil pressure Pc, a clutch oil temperature T0, and a duration of time t where the revolution speed difference ΔN is generated; the calculated temperature Tp is classified into a plurality of predetermined temperature intervals; the cumulative duration of time is measured for each of the predetermined temperature intervals, whereby a calculated temperature is within a corresponding interval; by applying Miner's law to a clutch disc endurance curve (a characteristic curve as to the disc lifetime expectancy in relation to the disc temperature), the remaining lifetime of the clutch, namely, the roadworthy remaining hours are calculated and displayed in a display. Thus, the patent reference 2 regards a failure of the clutch as foreseeable.
In general, Miner's law is a law by which a cumulative damage of a material is estimated according to the S-N characteristic curve (a fatigue endurance limit characteristic curve) of the material; whereby, S denotes the stress (a force per unit area) that works on or in the material, and N denotes the allowable maximum repetition numbers at the stress S; further, in the law of Miner, the material is regarded as damaged (fatigue-fractured) when the following equation holds:
            ∑              i        =        1            m        ⁢                  n        i                    N        i              =  1where Ni is the allowable maximum repetition numbers at the stress S=Si, and the stresses S1, . . . , Sm are repeated n1 times, . . . , nm times, respectively. In actually considering the S-N curve, the S-N curve may be regarded as a histogram (a bar chart) in which the vertical axis corresponds to the stress Sand the divided intervals thereof, while the lateral axis corresponds to the numbers N of allowable repetition of the stress; thereby, a plurality of bars of the histogram is placed in parallel to the lateral axis, each of the bars corresponding to a stress interval on the vertical axis.
In the applied Miner's law of the above reference 2, a Tp-t characteristic curve (a curve as to the clutch disc temperature in relation to the cumulative time duration) instead of the S-N characteristic curve is employed so as to obtain a cumulative damage law.
As described later, in this invention, a Q-N characteristic curve instead of the S-N curve is used so as to estimate the clutch use limit by use of a Miner's law, whereby Q is related to heat release (amount) in the clutch, and N relates to an allowable maximum repetition numbers as to the clutch slip operation when the slip operation is repeated at an heat release increment (interval) to which a Q level belongs; namely, a Q-N bar chart is taken into account as is the case of the above described S-N bar chart.
In a conventional wet clutch, the clutch being provided between an engine and a driven machine such as a forklift, and a clutch disc of the clutch being connected to or disconnected from a counter clutch disc of the clutch so that an input shaft is connected to or disconnected from an output shaft by charging or discharging hydraulic oil pressure working on the surfaces of the clutch discs, a criteria (an allowable use limit of a clutch) determining means treats with a plurality of factors to determine the lifetime of the clutch such as the frequency (and mode) of use as to the clutch, the energy level at which the clutch absorbs generated friction energy, and the surface temperature of the clutch disc; and, the clutch is controlled so that the level of each of the factors does not conflict with a corresponding allowable limit (a criteria).
In the technology of the patent reference 1, the clutch wear estimating means is provided so as to judge whether or not the clutch has worn, through the arithmetic operation result as to the slip of the clutch based on the input rotation speed and the output rotation speed; hereby, the arithmetic operation result as to the slip corresponds to the determining factor as to the clutch lifetime; the wear judgment is performed by detecting the slip, when the position of the actuator reaches to the predetermined a position predetermined by the actuator position detecting means.
In other words, the technology of the patent reference 1 uses the slip factor as to the clutch in determining the clutch lifetime; however, as described above, in determining the criteria as to the lifetime of the clutch (an allowable use limit of a clutch), there are several factors such as the frequency (and mode) of use as to the clutch, the energy level at which the clutch absorbs generated friction energy, and the surface temperature of the clutch disc; but not only the reference 1 dose not disclose how these factors effect on the lifetime of the clutch, but also the reference 1 dose not show any means by which an alarm is raised prior to the lifetime expiration, as well as that failure protection measures are adopted.
Further, as described above, in the patent reference 2, the clutch disc temperature Tp is calculated; the calculated temperature is classified into a plurality of predetermined temperature-intervals; the cumulative duration of time is measured per each of the predetermined temperature-intervals, whereby the calculated temperature is within the corresponding interval; by applying Miner's law to a clutch disc endurance curve (a characteristic curve as to the disc lifetime expectancy in relation to the disc temperature), the remaining lifetime of the clutch, namely, the roadworthy remaining hours are calculated and displayed in a display. Thus, the reference 2 regards a failure (damage) of the clutch as foreseeable. However, the reference 2 does not show how to establish the criteria as to the lifetime of the clutch (an allowable use limit of a clutch).